Smart antenna arrays, otherwise known as adaptive antenna arrays, comprise of low gain antennas connected to a combining/beamforming network. Smart antennas can provide enhanced coverage through range extension, hole filling and improved building penetration. By improving transmission and reception at the base station, the tolerable path losses can be increased whereby the range of the base station can be improved. By using smart antennas at the base station, initial deployment costs of a wireless network can be reduced: with the development in system usage, system capacity can be increased by adding additional cell sites and decreasing the range of existing base stations.
Digital beamforming, whereby smart antenna arrays are conveniently implemented, is considered as one of the most promising techniques for UMTS networks. Linear and planar antenna arrays are normally considered as candidate antenna types, since they produce low sidelobe levels with respect to other types of antenna such as the circular array. If a communications antenna array produces high sidelobes in the beampattern, it will cause strong interfering signals to mobile terminals in some directions, which can be a severe problem for downlink transmission.
In the R'99 UTRAN specification, a feedback mechanism was introduced to assist the transmit diversity technique employing two antennas. The concept is to transmit downlink signals of constant power from two different antennas using different scrambling codes to all the mobiles in the common channels and then to adapt the antenna weights for dedicated channels according to the feedback received from the intended mobile. Currently, there is great commercial interest in proposals to introduce feedback signalling to four antennas. Such feedback information enables adaptive beamforming for arrays with four elements. On the one hand, owing to the limited bandwidth for signalling, it is unlikely that any signalling mechanism for more than four antennas will be ever introduced. On the other hand, certain arrays such as circular arrays for cellular networks need at least eight antennas to achieve significant array gain. Also, it is conceivable that linear antenna arrays consisting of more than four elements will be used in future UTRAN products.